Enterobacteriaceae are a large and diverse family of bacteria, often classified using Gram staining. This fundamental microbiology tool distinguishes bacteria into two major groups. Enterobacteriaceae are Gram-negative bacteria. This classification stems from specific structural features of their cell walls, which interact with Gram stain reagents characteristically. Understanding this classification is important for identifying these microorganisms and predicting their behavior, particularly in medical and environmental contexts.
Understanding Gram Staining
Gram staining is a laboratory technique developed by Hans Christian Gram in 1884, used to differentiate bacterial species into two large groups: Gram-positive and Gram-negative. The process involves several steps that exploit differences in bacterial cell wall composition. Initially, a crystal violet stain is applied, coloring all bacterial cells purple.
Next, Gram’s iodine, a mordant, is added, forming a large, insoluble crystal violet-iodine complex within the cell. Decolorization with alcohol or acetone follows. Gram-positive bacteria, with their thick peptidoglycan layer, retain this complex, appearing purple.
In contrast, Gram-negative bacteria possess a much thinner peptidoglycan layer and an outer membrane rich in lipids. The alcohol decolorizer dissolves this lipid-rich outer membrane, allowing the crystal violet-iodine complex to wash out of the cell. A counterstain, typically safranin, is then applied, staining the now colorless Gram-negative cells pink or red. This distinct coloration serves as a primary identifier in bacteriology.
Defining Enterobacteriaceae
The Enterobacteriaceae represent a large family of bacteria, encompassing over 30 genera and more than 100 species. These microorganisms are characterized by their rod-shaped morphology, measuring 1 to 5 micrometers in length. They are non-spore-forming.
They are facultative anaerobes, able to grow in the presence or absence of oxygen. All members ferment glucose and reduce nitrates to nitrites. These features underscore their adaptability and widespread presence.
Common Examples and Habitats
Many members of the Enterobacteriaceae family are commonly encountered in diverse environments, from the human body to soil and water. Escherichia coli (E. coli) is a well-known example, frequently found as a normal inhabitant of the lower intestines of humans and other warm-blooded animals. While many E. coli strains are harmless, some are pathogenic and can cause intestinal or extraintestinal infections.
Salmonella species are another group within this family, commonly residing in the intestines of birds, reptiles, and mammals. These bacteria are often linked to foodborne illnesses, transmitted through contaminated food and water. Klebsiella species, particularly Klebsiella pneumoniae, are found in the normal flora of the mouth, skin, and intestines, and also exist in soil and aquatic environments.
Significance of Gram-Negative Classification
The Gram-negative classification of Enterobacteriaceae is important, particularly concerning health and medical treatment. A defining feature of Gram-negative bacteria is their outer membrane, which contains lipopolysaccharide (LPS).
This LPS, also known as endotoxin, is a component of the outer membrane and contributes to the bacterium’s structural integrity. The outer membrane acts as a protective barrier, limiting the entry of certain antibiotics and other harmful substances into the bacterial cell.
This inherent barrier, along with efflux pumps that actively expel antibiotics, contributes to the natural resistance of Gram-negative bacteria to several antimicrobial agents. The lipid A component of LPS can trigger a strong immune response in the host, leading to symptoms like fever and inflammation. In severe cases, LPS release upon bacterial cell lysis can lead to septic shock, a life-threatening condition. This cell wall structure and the presence of LPS help explain the challenges in treating infections caused by Enterobacteriaceae.